1. Field of the Invention
The invention relates to joining of advanced superalloy components during fabrication or repair. In some embodiments, the invention relates to surface repair of superalloy turbine blades and vanes in steam or gas turbines, by use of studs that are affixed to a repaired substrate by electric arc heating of the stud and substrate to melt the surface of each, followed by driving the stud into the substrate under pressure, in a manner that does not significantly reduce mechanical structural or material properties of the joined components.
2. Description of the Prior Art
“Structural” repair of gas turbine or other superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving properties, such as strength, that are close to the original manufacture component specifications (e.g., at least seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced surface cracks, so that risk of further cracking is reduced, and the blades are restored to original material structural and dimensional specifications.
Structural repair or new fabrication of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgic properties of the finished blade material. For example, a superalloy having more than 6% aggregate aluminum or titanium content, such as CM247 alloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content X-750 superalloy. The finished turbine blade alloys are typically strengthened during post casting heat treatments, which render them difficult to perform subsequent structural welding. Currently used welding processes for superalloy structural fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding rod or other filler material added. When a blade constructed of such a material is welded with filler metal of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties comparable to a new component.
A past attempt to perform traditional “spot” electric resistance welding of superalloys, in a heat resistance joining apparatus, by passing current between compressed electrodes into a pair of abutting superalloy components also caused solidification cracking within the weld zone. Alternative superalloy welding processes, including laser micro cladding with chill fixtures, welding in so-called “hot” boxes at elevated temperatures, and inertia friction welding may still lead to post weld heat treatment strain age cracking. Other friction welding processes, such as friction stir welding, can reduce superalloy cracking propensity, but the employed welding apparatus has relatively limited tool life. The same cracking concerns occur during superalloy component fabrication, when separate components constructed of superalloy material are joined by welding processes.
Given the shortcomings of superalloy structural repair welding, often the only commercially acceptable solution is to scrap damaged turbine blades that require structural repair, because past experience has shown limited success of such structural repairs. Thus repairs have been limited to those that have in the past been proven to be performed successfully by alternative superalloy welding processes described above, or by cosmetic welding, employing more ductile welding filler materials with reduced structural strength.
Thus, a need exists in the art for a method for performing structural joining or repairs on surfaces of superalloy components, such as turbine vanes and blades, so that subcomponents can be joined; or those structural cracks and other surface defects can be repaired.
Another need exists in the art to increase successful rates of structural repairs on surfaces of superalloy components, such as turbine vanes and blades, so that damaged blade scrap rates can be reduced.
Yet another need exists in the art for a method for performing structural joining or repairs on surfaces of superalloy components, such as turbine vanes and blades, with proven, repeatable repair techniques and machinery that do not require complex welding or post-repair heat treatment procedures.